1. Field of the Invention
This invention relates generally to navigational systems for use with ships or aircraft and more particularly to an aircraft or ship navigation system which uses packet radio technology to remotely report position information.
2. Description of the Prior Art
Prior art methods of remotely determining the position of ships or aircraft normally involve the use of systems with a beacon on the ship or aircraft whose position is being tracked and then reported, or radar systems which track a ship or aircraft by means of microwave energy reflected from the ship or aircraft being tracked. Other methods involve multilateration of a beacon signal from multiple receiving sites. Using these prior art methods, a complex and expensive ground station sends interrogation signals to ships or aircraft being tracked. Return signals indicate ranges to the beacon on the ship or aircraft whose position is being tracked. Actual geographic or relative position of the aircraft or ship is then calculated by computers inherent within these systems. These systems tend to be complex, physically large, expensive and are not easily deployed.
Systems used for remote position monitoring/tracking of ships and aircraft in weapons test and evaluation applications are typically radars and multilateration tracking systems. Test ranges incorporate various types of radar for surface and air surveillance or precision tracking of vehicles under test. These radars are normally shore-based and their coverage does not extend above the horizon. Multilateration systems can extend their coverage over the horizon only if a complex and expensive transponder is installed in the unit to be tracked as well as aboard a relay aircraft.
Packet systems have been known for several years: see for example "Computer Networks" by Andrew S. Tanenbaum, published by Prentice Hall (1981) and "Advances in Packet Radio Technology" by R. E. Kahn et. al., Proc. IEEE, Vol. 66 (November 1978), pages 1468-1496. A packet radio system is a data communications radio network comprising a plurality of stations.
Generally, packet radio communication systems include a plurality of stations each covering a respective zone. A data message to be communicated is divided into discrete segments of fixed length, called "packets". Packets are transmitted from a station of origin to a destination station and if the packets are received by the destination station without error, an acknowledgement is provided by the destination station. Thus, two way communication may be accomplished by two or more stations within a network.
Specifically, packet radio communications systems may have a central station that administers a plurality of remote stations each covering a respective zone. In response to a polling packet from the central station, data packets are assembled at the remote stations and transmitted to the central station. When the packets arrive at the central station, the central station transmits an acknowledgement of that fact.
Packet radio systems have many uses in the communications field, such as, providing mobile battlefield data users with a common communications service which is comparable in terms of service and reliability to a static system. U.S. Pat. No. 4,989,204 to T. Shimizu et. al. is illustrative of a packet radio communication system which provides for mobile stations and may be used in tactical mobile areas of a forward battlefield. However, packet radio technology has not been utilized to remotely report aircraft or ship locations.